To understand the properties of timber,* it is necessary to be familiar with its structure which is altogether different from that of any other material of construction. Under a magnifying glass a piece of iron, for example, appears to be practically homogeneous. On the other hand, a piece of timber appears as a network with openings of various shapes and sizes. Fig. 16 shows the general appearance of the end grain of a piece of timber. The various openings visible in this photograph are the cavities of the different types of cells which are formed during the growth of the tree. Each type of cell has its own particular function in the growing tree. Before trying to understand the details of this structure, it will be helpful to consider the various parts of a tree and the way in which it grows.
Fig. 17 illustrates a portion of a cross-cut end of a log. In the centre is the pith, and around it in order, lie the truewood, the sapwood and the bark.
The sapwood and the truewood are divided into what are commonly known as growth rings,† Between the sapwood and the bark, there is a very thin layer—not visible in the illustration—called the cambium layer. What is the significance of these various layers and how do they come into existence?
T H E G R O W T H O F T H E T R E E
Growth in Height
The increase in the height of any seedling or tree, or in the length of a branch, is due to the division and growth of numerous special cells at the extreme tip. The subsequent elongation of the growing tip is the only
length-* See D.F.P. Trade Circular, no. 3.
† To overcome the confusion between the terms heartwood and "heart" which latter, in Australia, is commonly applied to the central and frequently more or less decayed portion of the tree, the term truewood has been adopted to describe that sound portion of the tree between the sapwood and the heart. This term is especially applicable since, while the heart of many Australian trees is useless, the truewood provides the bulk (and in many cases all) of the usable timber.
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wise growth that occurs in the tree, and the cells which give rise to this growth are thin-walled and do not, in themselves, produce woody tissue. When the tree is young vertical growth is rapid but as it matures the growth slows down appreciably. A short way back from the growing tip some of the cells formed
Figure 16. Appearance of end grain of hardwood.
Figure 17. End section of a log, showing pith, truewood, sapwood and bark.
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undergo definite changes. Some of them, those on the outside, go to form the important cambium layer which is discussed more fully below.
Growth in Girth
As the young tree grows in height, there is a corresponding growth in girth, which is the result of the division and growth of the cells of the cambium layer. This very fine layer is composed of soft thin-walled cells which lie just beneath the bark and which extend all around the stem from the roots to within a short distance of the growing tip. By division they give rise, on the outside, to cells forming the inner living portion of the bark, and on the inside, to much thicker-walled cells which form the woody cylinder of the tree—the first wood cells so produced surround the soft primary cells of the pith. As time goes on, the stem gradually increases in thickness by the addition of new wood cells on the outside of those already formed. This is well illustrated in Fig. 18, which shows the pith, the wood cells formed during four years' growth, the position of the cambium layer and the bark. The cambium layer thus functions in the laying down of both the wood and the bark on the stem and branches of the tree.
The main thing to note is that growth in height occurs only at the growing tip by the division and growth of the special cells located there, while growth in girth is the special function of the cambium layer; the wood of the tree is formed by division and growth of the cells of this layer. Neither the wood cells nor the bark cells of the cambium layer influence growth in height.
Wood or bark produced at any particular height above ground remains always
at that height. This can be readily demonstrated by driving a large nail or spike into the trunk of a young tree at a definite height above ground. As the tree grows both in height and girth it will be found that the nail or spike remains at a constant height above the ground, although it will become gradually embedded in the trunk as the new wood cells are laid down around it.
Sources of Food
The wood cells by the cambium layer are of different types. When first formed, they are living cells and certain of them serve for the conduction of
water (containing some mineral salts) from the roots to the leaves. In the leaves, the green colouring matter, in the presence of sunlight, converts certain constituents of the air into sugars, starch and so on, which are suitable food for the growing cells of the tree.* The solutions which rise through the
sap-wood to the leaves are enriched there by the food material and then pass down the inner layers of bark cells, thus providing the growing cambium cells with materials for the formation of new wood and bark. Some of the food material on its way down also reaches the living wood cells just inside the cambium layer by means of special cells called medullary ray or ray cells. These are produced for this purpose and for food storage. They will be considered
later in detail.
Thus, there is a distinct upward movement of water in the living wood cells just beneath the cambium layer to the leaves; and a downward movement of food materials in solution, through the inner bark cells, to the roots—the food materials being used to supply the cells of the cambium layer. If the cambium layer is damaged, for example, by cutting through bark right around the stem at any height, the supply of materials from leaves to roots is inter-rupted and the tree will gradually die. This is what happens when a tree is
"ringbarked".
Growth Rings
The growth in girth of a tree is not usually regular throughout the year, but varies with the seasons. In cold climates, there is a complete cessation of growth during the autumn and winter. At the beginning of spring, growth is at a maximum and the wood produced is light in weight. Towards the end of spring and throughout the summer, the rate of growth diminishes until it almost or quite ceases in the winter. This means that, following the light, quickly grown spring wood with large open cells, the tree produces denser wood, consisting of thicker-walled cells. Repetition of this process year after year leads to the formation of definite yearly rings of growth, known in countries with a cold climate as annual rings. Each annual ring starts with the less dense spring wood, and ends with the denser summer wood (see photos 19 and 20).
In the more temperate climate of Australia, although similar growth rings may be produced, they do not always mark years of growth but simply
* F o r the chemical constituents of wood, see D.F.P. Trade Circular no. 28,
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climatic changes. Such rings should not, then, be referred to as annual rings, but as growth rings; and, if they are not true annual rings, the terms "early wood" and "late wood" should be substituted for spring wood and summer wood respectively. Growth rings of the eucalypts, for example, are not as well defined as true annual rings produced by trees grown in colder countries, and are sometimes very difficult to distinguish. In many tropical and sub-tropical trees, growth is more or less regular throughout die year, and no distinct growth rings are produced.
Growth rings are not, properly speaking, rings. They are sheaths or cones of wood, one surrounding the other, with the pith as a central axis.
M I C R O S C O P I C S T R U C T U R E
Figs. 19 and 20 illustrate two small blocks of wood highly magnified. It is noticeable that these two pieces differ considerably in their cell structure;
they have been chosen as representing two distinct classes into which prac-tically all timbers can be divided. Fig. 19 represents a piece of what is commonly known as "hardwood" or "pored" timber, and Fig. 20 a piece of
"softwood" or "non-pored" timber.
The terms hardwood and softwood are misleading. Many timbers falling into the Fig. 19 class are quite soft, and some timbers falling into the Fig. 20 class are comparatively hard.
Wood Containing "Vessels"—"Pored" Timbers
Into this group of timbers fall many of the woods common to timber users. The most important are the eucalypts, blackwood, oaks, maples, wal-nuts, ashes, beeches, willows, mahoganies, basswood and hickory. Fig. 19 shows a diagrammatic representation of the structure of these woods, the most outstanding feature being the presence of the large tubular wood vessels.
Running up the stem of the tree, these wood cells, when newly formed, serve to conduct solutions from the roots to the leaves. Each vessel, in the first place, is made up of cells joined end to end. The cross walls are broken down either completely or partially, to form long tubes each of which may be many yards long. Each such tube is spliced at each end on to a similar one, providing a continuous means of conduction from the roots to the leaves. It will be noticed in Fig. 19 that the vessels frequently occur in groups and that there are openings or pits in their side walls, allowing passage of solutions from one vessel to another or to other types of neighbouring cells. The vessels themselves, although sometimes difficult to distinguish, as in coachwood (Ceratopetalum apetalum), are frequently large enough to be seen easily with the naked eye, for example, in the oaks or mountain ash {Eucalyptus regnans). They appear then on the cross section of the stem as small holes or pores.
The distribution of the vessels in each growth ring gives rise to a further very useful division of the timbers of this class. In timbers such as the imported oaks and hickory, Australian red cedar (Toona australis) and white cedar of Queensland (Melia azedarach), the vessels are larger and more concentrated in the early or spring growth. Such timbers are called "ring
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pored" (see photo 2 0 ) . Practically all Australian pored timbers, however, fall into the second class known as "diffuse pored" because the pores are distributed or diffused more or less evenly throughout the early and the late wood.
Further examination of Fig. 19 shows numerous, small, thick-walled cells with small cavities (see also Fig. 16). These wood fibres, as they are termed,
The Structure of Hardwood
This drawing represents the cell struc-ture of 0 block of hardwood about 1-32 inch high. The portion TT corresponds to the top surface of o stump or end surface of a log, and RR corresponds to a surface roughly parallel to the radius of the log.
The hardwoods have specialised vessels or pores (P) for conducting sap. The pores vary in size and are visible in some species.
Most of the smaller cells are wood fibres which give hardwood its strength. T h e y usually have small cavities and relatively t h i c k walls, containing pits which allow a passage far sap to reach f r o m one cavity to another. The rays (R) are strips of short horizontal cells t h a t extend in a radial direction. They store and distribute f o o d . The annual ring (AR) is usually sharply defined. As a rule the springwood (S) is more porous than the summerwood ( S M ) formed later in the year.
A l l the cells In wood. Including pores, fibres, ray cells, etc., ore f i r m l y cemented together by a t h i n layer of lignin at t h e middle lamella ( M L ) .
The Structure of Softwood
This drawing represents t h e cell struc-ture of a m i n u t e block of softwood about 1-32 inch h i g h , like the drawing of the hardwood structure. The rectangular units w h i c h make up this surface are sections through various cells, mostly tracheids or water carriers, ( T R ) , t h e walls of which f o r m the b u l k of the wood substance.
Springwood cells (S) ore distinguishable f r o m the summerwood cells ( S M ) . The springwood growth is more r a p i d , and To-gether with t h e summerwood, make up a year's g r o w t h . The rays (R) store and distribute horizontally t h e food material.
The symbol SP indicates a simple pit, an u n t h i c k e n e d portion of the cell wall
Figure 19. Pored timber—Hardwood.
Figure 20. Non-pored timber—Softwood.
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are most important from the point of view of strength. Their length is great compared with their diameter, but they are smaller in every way than the vessels which are distributed between them. Like the vessels, the wood fibres have their length parallel to the height of the tree. Their sole function is the support of the tree. It will be noticed that the fibres in the late wood of the growth ring gradually become smaller in diameter and thicker-walled. In timbers such as many of the eucalypts, where the growth rings tend to be indistinct, this feature is not so apparent.
Another feature to notice in Figs. 16 and 19 is the presence of rows of cells whose length is at right angles to the length of the vessels and fibres, that is, they are horizontal in the trees. These are the medullary ray cells.
They have comparatively thin walls, are used for the storage of food materials, and retain life somewhat longer than the other cells. Rays, as groups of these cells are termed, vary greatly in size and number, and also in the number and arrangement of the individual cells in the group. They are useful features for distinguishing between certain timbers, and are some-times visible to the naked eye on a cross section, being more than of an inch thick in some true oaks, sheoaks and in the silky oaks. The silver grain of these timbers seen on longitudinal radial surfaces is due to these large rays. In eucalypts, on the other hand, the rays are often difficult to see on the cross section even with a hand lens. On a split radial or quartered face, however, they appear as a fine mottling or cross hatching.
Surrounding the vessels, although not evident in Fig. 19, there are frequently patches of "soft tissue" composed of thin-walled cells. This soft tissue often appears as bands which at first glance can sometimes be mistaken for growth rings. In coach wood (Ceratopetalum apetalum) and rose mahogany (Dyso-xylon fraseranum), the bands of soft tissue provide the "figure" (see photo 21); while in the Queensland black bean (Castanospermum australe) the large quantity of soft tissue surrounding the vessels makes them a prominent and contrasting feature of the figure in the timber (see photo 22).
Woods-Without "Vessels"—Non-pored Timbers
The common timbers falling into this class include the following: pines, firs, spruces, cypress pine, hemlock, Douglas fir, hoop pine, kauri pine, New Zealand white pine and so on. The outstanding feature of the timber of these species is the simplicity of their structure—see Fig. 20. There are no vessels and no wood fibres, but functioning for both and intermediate in size are cells of a single type, known as tracheids. Like the vessels, the tracheids have communication pits in their walls.
Some timbers in this class appear to have pores irregularly distributed on a cross section (see photo 19), but on microscopic examination these prove to be openings surrounded by a layer of very small cells and are quite distinct from vessels. These openings are known as resin canals and serve only as a means of disposal of resinous materials produced by the tree. Rays are present, but are never conspicuous; they function in the same way as in the woods containing vessels.
S A P W O O D AND T R U E W O O D
In the very young tree all the wood is sapwood. The first wood produced is made up of living cells, and these function largely in conducting water from the roots to the leaves. When the cells are first formed by the cambium layer they consist of thin-walled tubes of cellulose, a substance of which cotton is a pure form. The cells contain living matter, and grow rapidly, the walls becoming thicker and undergoing a change known as lignification, which adds considerably to their strength. Further changes go on over a long period. The living matter in the cells dies; tannins and other substances, some of which are coloured, escape from the dying cells and permeate the wood; and in some cases the cell cavities become blocked by outgrowths from the walls, or by gummy or resinous matter.
Thus, the inner layers of sapwood become converted into the less permeable and usually darker coloured truewood. Although this change is not accom-panied by an increase in strength, the materials deposited in the cells help to give resistance to the attacks of fungi and insects. This is why truewood is, as a rule, so much more durable than sapwood.
All the cells in the truewood are dead, they contain water, but do not serve in conducting solutions or water to the leaves. That is to say, the cells towards the inside of the sapwood die, and cease to function in conducting solutions, but new cells produced on the outside of the sapwood take up their work. The old cells then function simply by giving support to the tree. This continues year after year until the woody portion of the tree may consist almost entirely of dead cells which are surrounded by a zone of living wood cells. In some timbers this zone is very narrow but in others it may be quite wide. In certain species it has been found that little, if any, truewood is formed.
Sapwood, owing to the fact that its cells are more permeable, is much more readily penetrated by preservative fluids. Hence, if properly treated it can be rendered as durable, or even more durable than truewood. In such a case, being quite as strong as the truewood, it is really a superior article.
F I G U R E , T E X T U R E AND GRAIN
When describing the appearance of a particular wood species, the four features commonly quoted are colour, figure, texture and grain.
These features are of considerable importance in the grading and selection of timber, especially for furniture, joinery and similar purposes.
Figure
Figure refers to the pattern produced on longitudinal surfaces of wood resulting from one or a combination of three characteristics: (a) the arrange-ment and relative dimensions of the tissues, (b) the nature of the grain, and
Figure refers to the pattern produced on longitudinal surfaces of wood resulting from one or a combination of three characteristics: (a) the arrange-ment and relative dimensions of the tissues, (b) the nature of the grain, and